1. Field of the Invention
The present invention relates to a projection optical system, and more particularly, to an ultra wide angle zoom lens for a rear projection television (TV), which can be used in all fields irrespective of a thickness and display screen size of a TV system.
2. Discussion of the Related Art
Recently, with the increase in demand for large-size screens and high-definition images, projection systems that enlarge and project small images using a projection lens have gained popularity. The projection systems are roughly classified into a front projection system and a rear projection system depending on a direction of an image projected on a screen. The rear projection system has received much attention due to its advantageous ability of displaying relatively bright images even in bright surroundings.
An example of the rear projection system includes a projection TV. In the projection TV, a cathode ray tube (CRT) mode has been mainly used as a light source for displaying small images. However, it is difficult to construct a slim-size projection TV having the CRT mode due to a size of the CRT. For this reason, it is difficult to provide a large-size screen, a slim depth and the luminance required for high resolution in the projection TV.
To solve such problems, a projection TV based on a flat display, that can provide a large-size screen at a thin thickness, has been suggested.
Examples of the flat display include a liquid crystal display (LCD), a plasma display panel (PDP), a field emission display (FED), and an electro-luminescence (EL) device.
Among them, the projection TV using the LCD projects light emitted from a light source onto the LCD and displays an image of a liquid crystal panel on a screen using a projection lens system. Since the image is enlarged and projected on the screen using the liquid crystal panel of high image quality and a small size, a large-size screen image can be easily provided along with a slim-sized projection system. Moreover, the projection display system based on a liquid crystal panel can provide relatively high resolution and high luminance compared to the CRT. Therefore, a large-sized screen can be provided.
The projection display system based on a liquid crystal panel includes an optical engine, a total reflection mirror and a screen. The optical engine includes an illuminating system, a liquid crystal panel and a projection lens system.
In the projection display system, the illuminating system generates light and irradiates the generated light onto the liquid crystal panel. The liquid crystal panel displays an image by controlling transmissivity of incident light from the lighting system in accordance with an image signal. The projection lens system enlarges and projects the image from the liquid crystal panel and displays the image on the screen, thereby enabling a viewer to view the image displayed on a screen.
In this case, the image projected by the projection lens system is totally reflected by the total reflection mirror to change a light path. The image reflected by the total reflection mirror moves through the changed light path to the screen and then is displayed on the screen. If the projected image is directly projected from the rear of the screen without any change of the light path by the total reflection mirror, the thickness of the system becomes great. Accordingly, it is desirable to change the light path using the total reflection mirror so as to reduce the thickness of the system.
FIG. 1 is a schematic diagram illustrating a related art projection display system based on a liquid display panel.
Referring to FIG. 1, the related art projection display system includes an illuminating system, a liquid crystal panel and a projection lens system. The illuminating system includes a light source having an elliptical or parabolic reflection mirror 10 and a lamp 12, first and second fly eye lenses (FEL) 22 and 24, a polarizing beam splitter array (PBS array) 26 and a condensing lens 28 arranged between the light source 12 and a first dichroic mirror 30. The liquid crystal panel includes dichroic mirrors 30 and 34 and total reflection mirrors 32, 38 and 42. The projection lens system includes a dichroic prism 46 and a projection lens 48. Additionally, the projection display system further includes first and second relay lenses 36 and 40, Red/Green/Blue (RGB) liquid crystal panels 44R, 44G and 44B, and a screen 50.
An operation of the projection display system will now be described in detail with reference to FIG. 1.
Referring again to FIG. 1, visible lights emitted from the lamp 12 are reflected by the elliptical or parabolic reflection mirror 10 and move to the first FEL 22. The first FEL 22 divides incident lights on a cell basis and focuses the divided lights upon respective cells of the second FEL 24. The second FEL 24 converts incident lights into parallel lights and sends them to the PBS array 26. The PBS array 26 splits incident lights into linearly polarized lights having the same axis, namely a P-wave and an S-wave, and then converts the P-wave into an S-wave by a wavelength plate attached partially on its rear surface.
Accordingly, incident lights are all converted into linearly polarized lights of one direction, namely S-waves, whereby nearly all the lights emitted from the light source are inputted to the RGB liquid crystal panels 44R, 44G and 44B. At this time, the condensing lens 28 condenses lights outputted from the PBS array 26 to the liquid crystal panels 44.
The first and second dichroic mirrors 30 and 34 are arranged between the condensing lens 28 and the RGB liquid crystal panels 44R, 44G and 44B.
That is, the first total reflection mirror 32 and the red liquid crystal panel 44R are arranged to one side of the first dichroic mirror 30, and the second dichroic mirror 34 is arranged to another side of the first dichroic 30.
The green liquid crystal panel 44G is arranged to one side of the second dichroic mirror 34, and the first relay lens 36, the second total reflection mirror 38, the second relay lens 40, the third total reflection mirror 42 and the blue liquid crystal panel 44B are arranged to another side of the second dichroic mirror 34.
The dichroic prism 46 is arranged on three surfaces of the RGB liquid crystal panels 44R, 44G, and 44B, and the projection lens 48 and the screen 50 are arranged to the remaining side of the dichroic prism 46.
At this time, the total reflection mirror 32 totally reflects red light from the first dichroic mirror 30 to thereby transmit the reflected red light to the red liquid crystal panel 44R. Here, the red liquid crystal panel 44R is a transmissive LCD, which transmits the red light transmitted by the first total reflection mirror 32 to the dichroic prism 46.
Also, the second dichroic mirror 34 reflects a green light out of the lights having passed through the first dichroic mirror 30 while transmitting a blue light out of the lights having passed through the first dichroic mirror 30. Accordingly, the green light reflected by the second dichroic mirror 34 is transmitted to the green liquid crystal panel 44G. Here, the green liquid crystal panel 44G is a transmissive LCD, which transmits the green light transmitted by the second dichroic mirror 34 to the dichroic prism 46.
Also, the blue light having passed through the second dichroic mirror 34 is transmitted through the first relay lens 36, the second total reflection mirror 38, the second relay lens 40 and the third total reflection mirror 42 to the blue liquid crystal panel 44B. In this case, the first and second relay lenses 36 and 40 are field lenses, which delay a focus of the blue light prior to transmission of the blue light to the blue liquid crystal panel 44B. Here, the blue liquid crystal panel 44B is a transmissive LCD, which transmits the blue light transmitted by the third total reflection mirror 42 to the dichroic prism 46.
In this manner, the RGB liquid crystal panels 44R, 44G and 44B respectively reproduce a light image of each color by means of the received R, G and B lights in accordance to an image signal. In this case, an S-wave inputted to each of the RGB liquid crystal panels 44R, 44G and 44B is converted into a P-wave by each liquid crystal panel.
In this manner, the dichroic prism 46 combines received red, green and blue lights by using three-color image information from the RGB liquid crystal panels 44R, 44G and 44B. That is, the dichroic prism 46 reflects red and blue lights toward the projection lens 48 while transmitting a green light to the projection lens 48, thereby combining red, green and blue images.
Thereafter, the projection lens 48 enlarges the images from the dichroic prism 46 to then project the enlarged images on the screen 50.
The so-constructed projection display system can be small and lightweight.
Additionally, research for reducing the thickness of the projection display system while increasing its screen size, have been conducted. To make the size of the screen large and reduce the thickness of the system, it is necessary to decrease a projection distance between the screen 50 and the projection lens 48.
For this, the projection lens system includes a first lens group having a positive refractive power, and a second lens group having a negative refractive power. At this time, a total reflection mirror for changing the light path is disposed between the first lens group and the second lens group to form an “L” shaped projection lens system, whereby the thickness and the height of the system can be reduced.
However, in the “L” shaped projection lens system, the negative refractive power of the second lens group is great in order to obtain a short projection distance. Accordingly, aberrations such as distortion, coma, and astigmatism greatly occur.